Mobile communication terminal

ABSTRACT

A reference pattern comparing means 14 detects the number of error bits, using a reference pattern data portion embedded in the received signal. An error correcting means 15 detects the number of the corrected bits in the user data portion in the signal. The bit error rate measuring means 16 measures the bit error rate, on the basis of the number of the error bits and the corrected bits. The electric field level measuring means 17 measures the level of the electric field of the received signal. The electric field correcting means 18 corrects the level of the electric field, on the basis of the bit error rate. The control of the handing over of the radio communication channel is carried out, on the basis of the corrected electric field.

TECHNICAL FIELD

The present invention relates to a mobile terminal for use in a mobilecommunication system. Specifically, the present invention relates to acontrol of the handing over of the communication channel so as tocontinue the communication, when a mobile terminal moves from a basestation cell of a current base radio station, which is communicatingwith the mobile terminal, to the base station cell of another base radiostation. In a handing over procedure, a supplemental communicationchannel is provided to the communication channel of the new base radiostation.

BACKGROUND ART

In a mobile communication system, when a mobile terminal moves from thebase station cell of a communicating base radio station into the cell ofa neighboring base radio station, it is necessary to hand over thecommunication channel to the neighboring base radio station, and toprovide a supplemental channel to the neighboring base radio station soas to continue the communication.

Japanese Patent Application JP-A-9-37327 discloses an example of thecontrol of handing over in the prior art mobile communication system.FIG. 1 is a schematic diagram of the control of handing over disclosedtherein. Reference numerals 1 a, 1 b, 1 c denote base radio stations,each of which covers a base station cell 2 a, 2 b, 2 c. Referencenumeral 3 denotes a mobile terminal, which is moving in the area of thebase station cells 2 a, 2 b, and 2 c. The mobile terminal iscommunicating with one of the base radio stations.

FIG. 2 is a flow chart showing the control of the handing over. Atfirst, the mobile terminal 3 measures the level and the qualities ofcommunication of the input signal received from the currentlycommunicating base radio station 1 a (step ST 1). The qualities ofcommunication include, for example, a bit error rate and a frame errorrate. Then the mobile terminal judges whether the received input leveland the qualities of the communication are higher than a predeterminedvalue (step St 2). When they are higher than predetermined values, theprocedure returns to the step St 1. On the other hand, when they areless than the predetermined values, the mobile terminal measures thelevel and the qualities of communication of the input signals receivedfrom the other base radio stations 1 b, 1 c. (step St 3).

When the level of the input signal and the qualities of communication ofthe input signal received therefrom is higher than a predeterminedvalue, the mobile terminal measures the tendency of changing rate of thelevel and the qualities of communication of the input signal receivedfrom the base radio stations, and judges whether they are increasing ordecreasing (step ST 4).

Then the mobile terminal judges whether there are base stations, thetendency of changing rate of the level or the qualities of communicationof the input signal therefrom is increasing (step ST 5). When there areat least one base radio station, which has such an increasing tendency,the mobile terminal judges whether a plurality of the radio basestations have such an increasing tendency or only one base radio stationhas such an increasing tendency (step ST 6).

When only one base station has such an increasing tendency, thecommunication is handed over to the base radio station (step ST 7). FIG.1 (a) shows such a situation. In this case, the communication channel ishanded over to the base station cell 2 c of the base radio station 1 c.On the other hand, when a plurality of base radio stations have anincreasing tendency, the communication channel is handed over to thebase radio station, the level of the input signal or the qualities ofcommunication of the input signal received therefrom is the lowest amongthem (step ST 8). FIG. 1 (b) shows such a situation. In this case, thecommunication channel is handed over to the base station cell 2 c of thebase radio station 1 c.

When it is judged that no base radio station has such an increasingtendency in the step ST 5, the communication channel is handed over tothe base station cell of the base radio station, the level of the inputsignal or the qualities of communication of the input signal receivedtherefrom is the highest (step ST 9). FIG. 1(c) shows such a situation.In this case, the communication channel is handed over to the basestation cell 2 c of the base radio station 1 c.

Japanese patent application, JP-A-6-292258, discloses another example ofthe control of handing over in a mobile communication system, in whicheach of the mobile terminals in the mobile communication system has afunction to generate a data for measuring the bit error rate (BER) ofthe input signal received by the mobile terminal. Each of the base radiostations has a function to receive the data to measure the bit errorrate BER, so that the communication channel is handed over to the basestation cell of the base radio station, the bit error rate BER of whichis the smallest.

In general, it is known that a plurality of communication paths appearbetween a base radio station and a mobile terminal due to thediffraction and/or the reflection of the electromagnetic wave, and amulti-fading effect due to a plurality of communication paths isobserved. In such a circumstance, the level of the input signal receivedby the mobile terminal shows a Rayleigh's distribution, as shown in FIG.3. For avoiding the influence of the fading effect, a temporal meanvalue of the instantaneous level of the input signals in a certain timeinterval is often adopted as the level of the input signal. However,because the Rayleigh's distribution is a function of the spatialposition, the change of the instantaneous measured level of the inputsignal depends on the state of the mobile terminal, for example, thevelocity and direction of the movement. Therefore, it is difficult toeliminate the influence of the fading effect using such a temporal meanvalue.

When a moving vehicle, in which a mobile terminal is equipped, moves ata high velocity, it is necessary to shorten the smoothing time of theinput signal, and the temporal resolution of the input signal has to beincreased, in order to follow the rapid change of the level and/or thequalities of communication of the input signal. On the other hand, whenthe moving vehicle moves at a low velocity or is nearly stopping, thetime for smoothing the input signal has to be long so as to increase thepreciseness of the measurement. That is because a simultaneous actuationof a plurality of channels for handing over has to be prevented. Suchsimultaneous actuation can take place at a marginal area of a basestation cell of a base radio station, due to the chattering phenomenonof the input level or qualities of communication of the input signal.However, the mobile terminal of a mobile communication system in theprior art has no means to detect the velocity of the moving object.

When a moving vehicle, in which a mobile terminal is equipped, moves ata high velocity, it is necessary to shorten the smoothing time of theinput signal, and the temporal resolution of the input signal has to beincreased, in order to follow the rapid change of the level and/or thequalities of communication of the input signal. On the other hand, whenthe moving vehicle moves at a low velocity or is nearly stopping, thetime for smoothing the input signal has to be long so as to increase thepreciseness of the measurement. That is because a simultaneous actuationof a plurality of channels for handing over has to be prevented. Suchsimultaneous actuation can takes place at a marginal area of a basestation cell of a base radio station, due to the chattering phenomenonof the input level or qualities of communication of the input signal.However, the mobile terminal of a mobile communication system in theprior art has no means to detect the velocity of the moving object.

It is well known that the frequency of an electromagnetic wave measuredin a moving object is shifted compared to the frequency measured in astatic system due to the Doppler effect. The frequency f₀ measured in amoving system can be expressed as follows:

f ₀ =f−k·v  (1)

where f is the frequency measured in a static system, v is the velocityvector of the moving system, and k is the wave number vector of theelectromagnetic wave.

The clock in a mobile terminal is deviated from that of the base radiostation. Thus, in such a mobile communication system, the frequency ofthe mobile terminal is pulled to the frequency of the clock of the baseradio station so as to receive the electromagnetic signal from the baseradio station. Thus, the frequency deviation Δf of the measured signalincludes the frequency deviation f_(offset) between the clocks as wellas the frequency deviation due to the Doppler effect:

Δf=f _(offset) −k·v  2)

In a TDMA type communication system, a time slot is allocated to each ofthe communicating mobile terminals. The time slots allocated to aplurality of mobile terminals can overlap in the base radio station dueto the delay of the propagation of electromagnetic wave. For avoidingthe overlapping, the base radio station controls finely the signalsending timing from each of the mobile terminals, by setting a timealignment, which instructs the change of the sending timing from each ofthe mobile terminal. That is to say, a mobile terminal found in a farpoint from the base radio station begins to send signals at an advancedtiming compared to a mobile terminal found in a nearer point to the baseradio station

The starting time to send signals from a mobile terminal is determinedon the basis of the receiving time of the signal from the base radiostation, and is changed by the time alignment, which instructs how longthe sending timing shall be advanced. Therefore, the distance betweenthe mobile terminal and the base radio station can be detected from thetime difference between the receiving timing and the sending timing ofthe signal. When the mobile terminal is moving, the time alignment ischanged at every moment. And when the mobile terminal is nearlystopping, the time alignment is almost constant.

By the way, there is a relation between the proportion Eb/NO, which is aproportion of bit energy to noise density, and the bit error rate BER.An example of the relation is shown in FIG. 4. This relation dependsonly on the characteristics of the receiver, and does not depend on theenvironment. According to this relation, even when the electric field ofthe received electromagnetic wave is strong, if the noise contained inthe received signal is large, a small proportion Eb/N0 and a large biterror rate BER are observed.

The qualities of a communication channel can be substantially estimatedby the bit error rate BER. However, in a mobile communication system,the measurement of the bit error rate BER is difficult. Thus, theproportion Eb/N₀ is often employed in place of the bit error rate BER.However, in this case, even when the level of the received signal islarge, if the intensity of the interference wave or noise are stilllarger, and the mobile communication system has no means to measure themoving velocity of the moving vehicle, it is difficult to adjust thesmoothing time of the input signal, corresponding to the moving velocityof the moving vehicle. Thus, it is difficult to avoid the simultaneousactuation of a plurality of the handing over procedures.

The qualities of the communication channel can be evaluated according tothe level of the electric field of the received signal. In such a case,however, even when the level of the received signal is large, if theintensity of the interference wave or noise are still larger, the signalto noise ratio S/N decreases apparently. Thus, in such a case, thequalities of a communication channel is evaluated to be rather lowcontrary to the actuality.

An object of the present invention is to eliminate these problems.

Another object is to propose a mobile terminal for a mobilecommunication system, comprising a hand over controlling means, whichperforms a procedure to select the best base radio station, even whenthe mobile terminal is in a fading circumstance, so that the number ofthe handing over actuation times is reduced as least as possible, so asto avoid the deterioration of the qualities of communication and toreduce the traffic of the controlling signals.

DISCLOSURE OF THE INVENTION

In an embodiment of the present invention, the mobile terminal for amobile communication system comprises: an electric field level measuringmeans for measuring the level of the electric field of the signalreceived from the base radio station; a bit error rate measuring meansfor measuring the bit error rate of the signal received from the baseradio station; an electric field correcting means for correcting thelevel of the electric field measured by the electric field levelmeasuring means, on the basis of the bit error rate measured by the biterror rate measuring means. And the handing over between the radiocommunication channels is controlled on the basis of the electric fieldcorrected by the electric field correcting means.

According to this structure, the qualities of communication between themobile terminal and the base radio station after the handing over isimproved, and the continuous actuation of the handing over proceduresseldom takes place. Thus the increase of the traffic due to thecontinuous actuation of the handing over procedures can be prevented.

In another embodiment of the present invention, the mobile terminal fora mobile communication system further comprises: a reference patterncomparing means for comparing the reference pattern data portion, whichis embedded in the data received from the base radio station, with areference pattern stored in the mobile terminal, so as to detect thenumber of the error bits contained in the reference pattern dataportion; and an error correcting means for correcting the user dataportion in the received data and for detecting the number of thecorrected bits; wherein the bit error rate measuring means measures thebit error rate of the receive signal, on the basis of the number of theerror bits in the reference pattern data portion, which is measured bythe reference pattern comparing means, and the number of the correctedbits in the user data portion, which is detected by the error correctingmeans.

In another embodiment of the present invention, the mobile terminal fora mobile communication system comprises: an electric field levelmeasuring means for measuring the level of the electric field of thesignal received from the base radio station; a mobile terminal velocitymeasuring means for measuring the velocity of the mobile terminal withrespect to the base radio station. And the handing over between theradio communication channels is controlled on the basis of the velocitymeasured by the mobile terminal velocity measuring means and the levelof the electric field of the received signal, which is measured by theelectric field level measuring means.

As a result, the qualities of communication after the handing over isimproved. And a continuous actuation of the handing over proceduresseldom takes place. Thus, the increase of the traffic due to thecontinuous actuation of the handing over can be prevented.

In another embodiment of the present invention, the mobile terminal fora mobile communication system comprises: an electric field forecastingmeans for forecasting the level of the electric field of the signal tobe received in the future, on the basis of the level of the electricfield measured by the electric field measuring means and the movingvelocity of the mobile terminal with respect to the base radio station,which is measured by a mobile terminal velocity measuring means. Thehanding over between the radio communication channels is controlled,using the level of the electric field forecasted by the electric fieldforecasting means.

It is easy to perform a procedure to control the handing over to selectthe base radio station, the forecasted level of the electric field ofthe signal to be received therefrom is the highest, as the base radiostation to be handed over.

In another embodiment of the present invention, the mobile terminal fora mobile communication system further comprises: a Doppler frequencydetecting means for detecting the Doppler frequency of the signalreceived from the base radio station. And a mobile terminal velocitymeasuring means measures the moving velocity of the mobile terminal withrespect to the base radio station, on the basis of the Dopplerfrequency, which is detected by the Doppler frequency detecting means.

The fact that the Doppler frequency detected by the Doppler frequencydetecting means is “positive” means that the mobile terminal is movingtowards the base radio station, and the fact that the Doppler frequencyis “negative” means that the mobile terminal is moving in the directionto separate from the base radio station. The absolute value of theDoppler frequency is proportional to the moving velocity of the mobileterminal in the propagation direction of the electromagnetic wave.

The Doppler frequency can be measured without being influenced by thefading effect. Thus, it is possible to detect precisely whether themobile terminal is approaching to the base radio station.

In another embodiment of the present invention, the mobile terminal fora mobile communication system comprises: an automatic frequencycontrolling means, which introduces in itself the frequency of thesignal received from the base radio station. And the Doppler frequencydetecting means detects the Doppler frequency, from the frequencydeviation between the frequency introduced in by the automatic frequencycontrolling means and the clock frequency of the mobile terminal.

In another embodiment of the present invention, the mobile terminal fora mobile communication system further comprises: a propagation delaydetecting means for detecting the propagation delay of theelectromagnetic wave between the mobile terminal and the base radiostation; and a distance detecting means for detecting the distancebetween the mobile terminal and the base radio station, on the basis ofthe propagation delay detected by the propagation delay detecting means.And the mobile terminal velocity measuring means measures the velocityof the mobile terminal with respect to the base radio station, on thebasis of the time variation of the distance detected by the distancedetecting means.

In this case, the qualities of communication between the mobile terminaland the base station after the handing over is improved. And acontinuous actuation of the handing over procedures seldom takes place.Thus the increase of traffic due to the handing over can be prevented.

In another embodiment of the present invention, the mobile terminal fora mobile communication system comprises a timing comparator whichcompares the timing of the signal sending from the mobile terminal andthe timing of signal receiving from the base radio station so as todetect the difference between them. And the distance between the mobileterminal and the base radio station is measured, on the basis of thedetected difference between the timing of the signal sending from themobile terminal and the timing of signal receiving from the base radiostation. In this case, the detected distance between the mobile terminaland the base radio station is hardly influenced by the fading effect.Thus, it is possible to detect precisely whether the mobile terminal isapproaching to the base radio station or not.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the control of handing over.

FIG. 2 is a flow chart showing the control procedure of the handing overin the prior art.

FIG. 3 is an example of the electric field depending on the position,due to the fading effect.

FIG. 4 is a relation between the bit error rate BER and the proportionof the bit energy to noise density.

FIG. 5 is a block diagram of the mobile terminal for a mobilecommunication system according to the first embodiment of the presentinvention.

FIG. 6 is a flow chart of the handing over procedure in the firstembodiment of the present invention.

FIG. 7 is a block diagram of the mobile terminal for a mobilecommunication system according to the second embodiment of the presentinvention.

FIG. 8 is a flow chart of the handing over procedure in the secondembodiment of the present invention.

FIG. 9 is a detail of forecasting procedure in the handing overprocedure in the second embodiment of the present invention.

FIG. 10 is a block diagram of the mobile terminal for a mobilecommunication system according to the third embodiment of the presentinvention.

FIG. 11 is a flow chart of the handing over procedure in the thirdembodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Best modes for carrying out the present invention is explained below,referring to the accompanied drawings, for more precise explanation ofthe present invention.

Embodiment 1

FIG. 5 shows a block diagram of the mobile terminal for a mobilecommunication system according to the first embodiment of the presentinvention. In the drawing, reference numerals 1 a, 1 b, 1 c denote baseradio stations. A mobile terminal 3 of this mobile communication systemis moving in the area of base station cells covered respectively by thebase radio stations 1 a, 1 b, 1 c. These components correspond to thosereferred to the same reference numeral in FIG. 1.

A receiving means 11 sets a receiving frequency, and receives the signalfrom the base radio stations 1 a, 1 b, 1 c, then detects the receivedsignals. The signal detected by the receiving means 11 is demodulated bya demodulator 12. The demodulated data is separated into a referencepattern data portion and a user data portion by a demultiplexer 13. Areference pattern comparing means 14 compares the reference pattern dataportion, separated by the demultiplexer 13, with a reference patternstored in the mobile terminal to detect the number of error bits in thereference pattern data portion. An error correction means 15 correctserrors contained in the user data portion separated by the demultiplexer13 and counts the number of the corrected bits.

A bit error rate measuring means 16 calculates the bit error rate fromthe number of error bits, which are contained in the reference patterndata portion and are detected by the reference pattern comparing means14, and the number of the corrected bits in the user data portion, whichare corrected by the error correction means 15.

An electric field level measuring means 17 measures the level of theelectric field of the received signal, which is received and detected bythe receiving means 11. An electric field correcting means 18 correctsthe level of the electric field, which is measured by the electric fieldlevel measuring means 17, on the basis of the bit error rate calculatedby the bit error rate measuring means 16. A controller 19 actuates aprocedure for handing over the radio communication channel, on the basisof the electric field level corrected by the electric field correctingmeans 18.

Each of the mobile terminals 3 in the mobile communication systemcomprises these receiving means 11, demodulator 12, demultiplexer 13,reference pattern comparing means 14, error correcting means 15, biterror rate measuring means 16, electric field level measuring means 17,electric field correcting means 18 and controller 19.

The function of the embodiment 1 is explained below:

FIG. 6 is a flow chart of the handing over procedure in such a mobileterminal 3 in a mobile communication system. The function is explained,assuming that the base station cells covered by each of the base radiostations 1 a, 1 b, 1 c are overlapping at their marginal portions. And,the mobile terminal 3 is communicating with the base radio station lacurrently, and the mobile terminal 3 can receive also the control signalfrom the other base radio stations 1 b, 1 c.

In the mobile terminal 3, the receiving means 11 receives the signalfrom the base radio station 1 a and detects it. The electric field levelmeasuring means 17 measures the level of the electric field of thesignal, which is received and detected by the receiving means 11 (stepST11). The electric field level measured by the electric field measuringmeans 17 is compared with a predetermined value (step ST12). When thelevel is higher than the predetermined value, the procedure returns tothe step ST 11. On the other hand, when the level is less than thepredetermined value, the level of the electric field of a control signalfrom the base radio stations 1 b, 1 c is measured by the electric fieldlevel measuring means 17 (step ST13).

Meanwhile, the demodulator 12 in the mobile terminal 3 demodulates thesignal received and detected by the receiving means 11. And thedemultiplexer 13 separates the signal demodulated by the demodulator 12into a user data portion and a reference pattern data portion. Thereference pattern data portion is previously embedded into the signal soas to be used in the bit error rate detecting procedure. The referencepattern comparing means 14 compares the reference pattern data portionseparated by the demultiplexer 13 with a previously stored referencepattern, and counts the number of the error bits in the referencepattern data portion (step ST 14).

The error correcting means 15 corrects the error bits in the user dataportion separated by the demultiplexer 13, and detects the number of thecorrected bits (step ST 15). The bit error rate measuring means 16calculates the bit error rate from the number of the error bits in thereference pattern data portion, which is detected by the referencepattern comparing means 14, and the number of the corrected bits in theuser data portion, which is detected by the error correcting means 15(step ST 16).

The electric field correcting means 18 corrects the level of theelectric field of the control signal, which is received from the baseradio stations 1 b, 1 c, using the bit error rate calculated by the biterror rate measuring means 16 for each of the base radio stations (stepST17). The electric field level corrected by the electric fieldcorrecting means 18 is judged whether it is higher than a predeterminedvalue (step ST 18). When the level is less than the predetermined value,the procedure returns to the step ST 11. On the other hand, when thelevel is higher than the predetermined value, the controller 19 in themobile terminal 3 selects the base radio station (for example, baseradio station 1 c) which corresponds to the highest corrected electricfield level. The selected base radio station is reported to the currentbase radio station 1 a. A handing over procedure among the mobileterminal 3, the currently communicating base radio station 1 a, and theselected base radio station 1 c is actuated (step ST19).

In the embodiment 1, as explained above, when there are a plurality ofbase radio stations, which can be handed over, one base radio station isselected, according to the following procedures:

The level of electric field of the signal from each of the base radiostations is measured;

After demodulation of the signal, the bit error rate is calculated, onthe basis of the number of error bits which is detected using areference pattern data portion embedded in the received signal, and thenumber of corrected bits in the user data portion;

The level of the control signal from each of the base radio stations iscorrected, using the bit error rate;

A base radio station to be handed over is selected, using the correctedlevel of the control signal.

As a result, even when the level of the signal from a base radio stationis high, if the signal to noise ratio of the signal from the base radiostation is low due to overlapping obstacle waves etc, the handing overto such a base radio station can be prevented. Therefore, thedeterioration of the qualities of communication after the handing overcan be avoided. And when the deterioration of the qualities ofcommunication after the handing over can be avoided, it is possible toreduce the number of the handing over. Consequently, the stopping ofcommunication at a split second due to handing over and/or the increaseof traffic can be avoided.

Embodiment 2

FIG. 7 is a block diagram of a preferred embodiment of the presentinvention, which is different from the embodiment 1 shown in FIG. 5.Components identical to or corresponding to those in FIG. 5 are referredto the same reference numeral. And those explanations are omitted.

In the figure, an automatic frequency controlling means 21 introducesthe frequency of the signal from the base radio station, using thesignal received and detected by the receiving means 11. And a Dopplerfrequency detecting means 22 detects the Doppler frequency from theshift of frequency between the received signal introduced by theautomatic frequency controlling means 22 and the clock frequency of themobile terminal 3. Then a mobile terminal velocity measuring means 23measures the velocity of the mobile terminal 3 with respect to the baseradio station 1 a, on the basis of the Doppler frequency detected by theDoppler frequency detecting means 22.

An electric field forecasting means 24 forecasts the electric field inthe future, for example, at a time after t seconds (t is changeable), onthe basis of the level of the input signal, which is received by thereceiving means 11 and measured by the electric field measuring means17, and the moving velocity of the mobile terminal 3 with respect to thebase radio station, which is measured by the mobile terminal velocitymeasuring means 23.

The function of the embodiment 2 is explained below:

FIG. 8 is a flow chart of the handing over procedure in such a mobileterminal 3 in a mobile communication system. The function is explained,assuming that the base station cell covered by each of the base radiostations 1 a, 1 b, 1 c are overlapping at their marginal portions. And,the mobile terminal 3 is currently communicating with the base radiostation 1 a, and the mobile terminal 3 can receive also the controlsignal from the other base radio stations 1 b, 1 c. These assumptionscorrespond to that in the explanation of the function of the embodiment1.

Electric field level measuring means 17 in the mobile terminal 3measures the level of the electric field of the signal, which isreceived from the base radio station 1 a and detected by the receivingmeans 11 (step ST 21). The level of the electric field measured by theelectric field level measuring means 17 is judged whether it is higherthan a predetermined value (step ST 22). When it is higher than thepredetermined value, the procedure returns to the step ST 21. On theother hand, when the level is less than the predetermined value, theelectric field level measuring means 17 measures the level of thecontrol signals from the base radio station 1 b, 1 c received by thereceiving means 11 (step ST 23).

In the next step, it is judged how many base radio station there are,the level of the signal received therefrom by the receiving means 11 ishigher than the predetermined value (step ST 24). When there is no sucha base radio station, the procedure returns to the step ST 21. Whenthere is only one base radio station, the level of the signal therefromis higher than the predetermined value, the base radio station isselected as the base radio station to be handed over (step ST 25). Andwhen there are a plurality of base radio stations, the level of thesignal therefrom is higher than the predetermined value, the frequencyof the signal therefrom is introduced in the automatic frequencycontrolling means 21 (step ST 26). Then the automatic frequencycontrolling means 22 measures the Doppler frequency for each of thosebase radio stations, on the basis of the deviation of frequenciesbetween the clock signal of the base radio station, which is introducedin the automatic frequency controlling means 21, and the clock frequencyof the mobile terminal 3 (step ST 27).

In the next step, it is judged whether there is a base radio station,the Doppler frequency of which is positive (step ST28). When there is nobase radio station, the Doppler frequency of which is positive, thelevel of the signal from those base radio stations is forecasted by theelectric field forecasting means 24, which performs the routine RT1. Theroutine will be explained later.

On the other hand, when there is at least one base radio station, theDoppler frequency of the signal therefrom is positive, it is judged howmany base radio stations there are, the Doppler frequency therefrom ispositive (step ST 29). When there is only one base radio station, theDoppler frequency therefrom is positive, the base radio station isselected as the base radio station to be handed over (step ST 25).

When there are a plurality of base radio stations, the Doppler frequencyof signal therefrom is positive, the procedure goes to the routine RT1to forecast the level of the electric field of the signal from thosebase radio stations. Before entering the routine RT1, the base radiostations, the Doppler frequency of the signal therefrom is negative, areexcluded from the list of the candidate base radio stations that can bea base radio station to be handed over (step ST30).

In the routine RT1, the level of the electric field of the signal fromthe base radio stations are forecasted as follows. FIG. 9 is a flowchart of the procedure of forecasting in the routine RT1. The mobileterminal velocity measuring means 23 measures the moving velocity of themobile terminal 3 with respect to the candidate base radio stations, onthe basis of the Doppler frequency of the signal from those base radiostations (step ST 41). Then the electric field forecasting means 24forecasts the level, in the future after t seconds, of the electricfield of the signal from those base radio stations, on the basis of thecurrent electric field of those signal and the moving velocity of themobile terminal 3 with respect to those base radio stations (step ST42). And the base radio station, the forecasted electric field of thesignal received from which is the highest, is selected as the base radiostation to be handed over (step ST 42). Then the procedure exits fromthe electric field forecasting routine RT1.

Exiting from the electric field forecasting routine RT1, the controller19 of the mobile terminal 3 reports the selected base radio station 1 cto be handed over to the currently communicating base radio station 1 a.Corresponding to this report, a handing over procedure among the mobileterminal 3, the currently communicating base radio station 1 a, and theselected base radio station 1 c is actuated (step ST 31).

The fact that the Doppler frequency detected by the Doppler frequencydetecting means 22 is positive means that the mobile terminal 3 ismoving towards the base radio station. And the fact that the detectedDoppler frequency is negative means that the mobile terminal 3 is movingin the direction to separate from the base radio station. The absolutevalue of the detected Doppler frequency is proportional to the movingvelocity of the mobile terminal 3 in the propagation direction of theelectromagnetic wave from the base radio station. Because the Dopplerfrequency can be observed without being influenced by the fading effect,it is possible to determine exactly whether the mobile terminal 3 isapproaching to the base radio station or not.

In the embodiment 2, as explained above, when there are a plurality ofcandidate radio stations to be handed over, the Doppler frequency of thesignal from those base radio stations are detected to measure the movingvelocity of the mobile terminal with respect to each of the base radiostations, and the level of the electric field, in the future after tseconds, of the signal are forecasted, on the basis of the movingvelocity and the present level of the electric field. And the base radiostation, the forecasted electric field of which is the highest amongthose candidate base radio stations, is selected as the base radiostation to be handed over.

As a result, the area that the mobile terminal can move using the basestation cell of the same base radio station increases. Therefore, thenumber of times of the handing over decreases. Consequently, thestopping of communication at a split second due to the handing over aswell as the increase of the traffic of the controlling signal can beprevented. Furthermore, because the moving velocity of the mobileterminal is measured on the basis of the Doppler frequency, themeasurement is hardly influenced by the fading effect. And it ispossible to detect exactly whether the mobile terminal is approaching tothose base radio station.

Embodiment 3

FIG. 10 is a block diagram of the mobile terminal for a mobilecommunication system according to a preferred embodiment 3, which isdifferent from the first and second embodiments shown in FIGS. 5 and 7,of the present invention. Components in the figure identical to orcorresponding to those shown in FIG. 7 are referred to the samereference numerals. And their explanation is omitted.

The demodulator 12 demodulates the received signal. And a decoder 31decodes the signal demodulated by the demodulator 12, and extracts thecontrol data therefrom, which was sent to the mobile terminal 3 from thebase radio station. A sending timing changing means 32 changes thetiming of the signal sending of the mobile terminal 3, according to theinstruction from the base radio station, which is contained in thecontrol data. A sending means 33 modulates the data to be sent and sendsthe signals at a timing determined by the sending timing changing means32. A propagation delay detecting means 34 detects the delay ofpropagation between base radio stations and the mobile terminal. In thisthird embodiment, the propagation delay detecting means 34 compares thesending timing of the mobile terminal 3, which is determined by thesending timing changing means 32, and the receiving timing of the signalfrom the base radio station so as to detect the timing differencebetween them.

A distance detecting means 35 detects the distance between the mobileterminal 3 and the base radio station, on the basis of the timingdifference between the signal sending timing and the signal receivingtiming which is detected by the propagation delay detecting means 34.The time variation of the distance between the mobile terminal and thebase radio station, which is detected by the distance detecting means34, is stored in a measured distance storing means 36.

A mobile terminal velocity measuring means 23 measures the velocity ofthe mobile terminal 3 with respect to the base radio station, on thebasis of the changing rate of the distance between the mobile terminal 3and the base radio station. The principle of the distance detection ofthe mobile terminal velocity measuring means 23 in this embodiment isdifferent from that of the mobile terminal velocity detecting means 23in the second embodiment shown in FIG. 7, though they are referred tothe same reference numeral.

The function of the mobile terminal for a mobile communication systemaccording to the embodiment 3 is explained below:

FIG. 11 is a flow chart of the procedure of handing over in the mobileterminal 3 for a mobile communication, which is formed as shown in FIG.10. The function is explained, assuming that the base station cellscovered by each of the base radio stations 1 a, 1 b, 1 c are overlappingat their marginal portions. And, the mobile terminal 3 is currentlycommunicating with the base radio station 1 a, and the mobile terminal 3can receive also signals from the other base radio stations 1 b, 1 c.These assumptions correspond to that of the explanation of the functionof the embodiments 1 and 2.

In this embodiment, the decoding means 31 of the mobile terminal 3decodes the signal demodulated by the demodulator 12. When a controldata from the base radio station is contained in the decoded signal, thedecoding means 31 reports it to the sending timing changing means 32.When an instruction for changing the sending timing is contained in thedecoded control signal, the sending timing changing means 32 sets thesignal sending timing of the sending means 33, according to theinstruction.

The receiving means 11 receives and detects the signal from thecurrently communicating base radio station 1 a. The electric field levelmeasuring means 17 in the mobile terminal 3 measures the level of theelectric field of the received signal (step ST 51). It is judged whetherthe level of the electric field is higher than a predetermined value ornot (step ST 52). When the level is higher than the predetermined value,the procedure returns to the step 51. When the level is less than thepredetermined value, the receiving means 11 receives the control signalfrom the other base radio stations 1 b, 1 c, and the electric fieldlevel measuring means 17 measures the level of electric field of thereceived control signal from the other base radio stations 1 b, 1 c(step ST 53).

In the next step, it is judged how many base radio stations there are,the level of the electric field of the signal therefrom is higher than apredetermined value (step ST 54). When there is no base radio station,the level of the electric field thereof is higher than the predeterminedvalue, the procedure returns to the step ST 51. When there is only onebase radio station like that, the base radio station is selected as thebase radio station to be handed over (step ST 55). When there are aplurality of base radio stations like that, the propagation delaydetecting means 34 compares the receiving timing of signals from thebase radio stations 1 b, 1 c and the timing of signal sending of themobile terminal 3 so that the difference of the timings is detected(step ST 56).

Then the distances between the mobile terminal 3 and the base radiostations 1 b, 1 c are measured, on the basis of the difference of thesignal sending timing and the signal receiving timing. The measureddistances are sampled at every predetermined sampling interval, forexample, at every t seconds (t is variable), and are stored in themeasured distance storing means 36 (step ST 57).

The mobile terminal velocity measuring means 23 obtains the timevariation of the read out distances between the mobile terminal 3 andthe base radio stations 1 b, 1 c stored in the measured distance storingmeans 36. The mobile terminal velocity measuring means 23 judges whetherthere are base radio stations, the tendency of the time variation of thedistance, which corresponds to the moving velocity of the mobileterminal 3, is negative (step ST 58).

When there is no base radio station, the tendency of the time variationof the distance is negative, a routine RT 2 for forecasting the level ofthe electric field of the received signal is immediately carried out bythe electric field forecasting means 24.

The electric field level forecasting procedure in the routine RT 2 issubstantially identical to that in the routine RT 1 in the secondembodiment. The routine is carried out, following the flow chart shownin FIG. 9. However, the principle of calculation of the moving velocityof the mobile terminal 3 with respect to the base radio station in thestep ST 41 is different from that in the second embodiment.Specifically, the moving velocity is measured, in the third embodiment,on the basis of the time variation of the distance between the mobileterminal 3 and the candidate base radio stations, the data of which isstored in the measured distance storing means 36.

In the step ST 58, when there is at least one base radio station, thetendency of the time variation of the distance is negative, it is judgedhow many base radio stations there are, the tendency of the timevariation of the distance is negative (step ST 59). When there is onlyone base radio station, the tendency of the time variation of thedistance is negative, the base radio station is selected as the baseradio station to be handed over (step ST 55).

On the contrary, when there are a plurality of base radio stations, thetendency of the time variation of the distance is negative, theprocedure enters into the routine RT 2 for forecasting the level of theelectric field of the received signal. Before entering the routine RT 2,the base radio stations, the tendency of the time variation of thedistance between the mobile terminal and those base radio station ispositive, are cancelled from the list of the candidate base radiostations (step ST 60).

After exiting the routine RT 2 for forecasting the level of the electricfield of the received signal, the controller 19 of the mobile terminal 3reports the selected base radio station, for example, the base radiostation 1 c, to the currently communicating base radio station 1 a.Corresponding to this report, a handing over procedure among the mobileterminal 3, the currently communicating base radio station 1 a and thebase radio station 1 c to be handed over is actuated (step ST 61).

The receiving timing of the signal, which was sent from the base radiostation in order that the propagation delay detecting means 34 comparesthe receiving timing of the signal with the sending timing of the signalfrom the mobile terminal 3, can be measured without being influenced bythe fading effect. Therefore, also the distance between the mobileterminal and the base radio station, which is detected by the distancedetecting means 35, is hardly influenced by the fading effect. As aresult, it is possible to detect precisely whether the mobile terminalis approaching to the base radio station or not.

In the third embodiment, as explained above, when there are a pluralityof candidate base radio stations to be handed over, the moving velocityof the mobile terminal with respect to the base radio station ismeasured, on the basis of the difference between the signal receivingtiming and the signal sending timing, which is adjusted by aninstruction from the base radio station; then the level of the electricfield of the signal in the future after t seconds is forecasted, on thebasis of the moving velocity and the present level of the electric fieldfrom the base radio station; and the base radio station, the forecastedlevel electric field of the signal therefrom is the highest, is selectedas the base radio station to be handed over.

As a result, the area that the mobile terminal can move using the basestation cell of one base radio station increases, therefore, the numberof the times of the handing over decreases. Consequently, the stoppingof communication at a split second due to the handing over as well asthe increase of the traffic of the controlling signal can be reduced.The moving velocity of the mobile terminal is measured on the basis ofthe receiving timing of the signal, therefore, the measurement is hardlyinfluenced by the fading effect. And it is possible to detect exactlywhether the mobile terminal is approaching to the base radio station.

INDUSTRIAL APPLICABILITY

As explained above, in the handing over procedure in the mobile terminalfor a mobile communication system according to an embodiment of thepresent invention, the level of the electric field of the signal fromthe base radio stations is measured, and after demodulating the signal,the bit error rate of the demodulated signal is measured. The base radiostation, the level of the electric field of the signal sent therefrom islarge and the bit error rate of the signal demodulated from the signalsent therefrom is low, is selected as the base radio station.

As a result, the qualities of communication between the mobile terminaland the base radio station after the handing over is improved. And acontinuous actuation of the handing over procedures seldom takes place.Thus, the increase of the traffic due to the handing over can beprevented. The handing over procedure is hardly influenced by the fadingeffect, thus, it is possible to detect precisely whether the mobileterminal is approaching to the base radio station or not. Consequently,a large advantage can be obtained, when the mobile terminal for a mobilecommunication system according to this embodiment of the presentinvention is used in the control of the handing over for changing thebase radio station.

In the mobile terminal for a mobile communication system according to anembodiment of the present invention, the level of the electric field ofthe signal from the base radio station is measured, and the Dopplerfrequencies of the signal received from the base radio stations aremeasured, on the basis of the introduced frequency of the signal fromthe base radio station, so that the moving velocity of the mobileterminal with respect to the base radio station is calculated; then thelevel of electric field in the future is forecasted, on the basis of thecalculated moving velocity of the mobile terminal and the present levelof the electric field of the signal received from the base radiostation; and the base radio station, the forecasted level of theelectric field of the signal therefrom is the largest, is selected asthe base radio station to be handed over.

As a result, the qualities of communication between the mobile terminaland the base radio station after the handing over is improved. And acontinuous actuation of the handing over procedures seldom takes place.Thus, the increase of the traffic due to the continuous handing over canbe prevented. The distance, according to the present invention, ishardly influenced by the fading effect, therefore, it is possible todetect precisely whether the mobile terminal is approaching to the baseradio station or not. Consequently, a large advantage can be obtained,when the mobile terminal for a mobile communication system according tothe this embodiment of the present invention is used in the control ofthe handing over for changing the base radio station.

In the mobile terminal for a mobile communication system according to anembodiment of the present invention, the level of the electric field ofthe signal from the base radio station is measured, and the delay ofpropagation of the electromagnetic wave between the mobile terminal andthe base radio station are measured so that the distance between themobile terminal and the base radio station is calculated, on the basisof the delay of propagation; then the moving velocity of the mobileterminal with respect to the base radio station is calculated, on thebasis of the time variation of the distance between the mobile terminaland the base radio station; and the level of electric field in thefuture is forecasted, on the basis of the calculated moving velocity ofthe mobile terminal and the present level of the electric field of thesignal received from the base radio station; and finally the base radiostation, the forecasted level of the electric field of the signaltherefrom is the largest, is selected as the base radio station to behanded over.

As a result, the qualities of communication between the mobile terminaland the base radio station after the handing over is improved. And acontinuous actuation of the handing over procedures seldom takes place.Thus, the increase of the traffic due to the handing over can beprevented. A large advantage can be obtained, when the mobile terminalfor a mobile communication system according to this embodiment of thepresent invention is used in the control of the handing over forchanging the base radio station.

When the distance between the mobile terminal and the base radio stationis calculated from the difference between the signal receiving timingand the signal sending timing, which is adjusted by the instruction fromthe base radio station, because the measured distance is hardlyinfluenced by the fading effect, thus it is possible to detect preciselywhether the mobile terminal is approaching to the base radio station ornot. Consequently, a large advantage can be obtained, when the mobileterminal for a mobile communication system according to this embodimentof the present invention is used in the control of the handing over forchanging the base radio station.

What is claimed is:
 1. A mobile terminal for a mobile communicationsystem, said mobile terminal comprising: an electric field levelmeasuring means for measuring the level of the electric field of thesignal received from the base radio station; a bit error rate measuringmeans for measuring the bit error rate of the signal received from thebase radio station; an electric field correcting means for correctingthe level of the electric field measured by the electric field levelmeasuring means, on the basis of the bit error rate measured by the biterror rate measuring means; and a controller for controlling the handingover between radio communication channels, on the basis of the electricfield corrected by the electric field correcting means.
 2. The mobileterminal according to claim 1, further comprising: a reference patterncomparing means for comparing a reference pattern data portion, which isembedded in the data received from the base radio station, with areference pattern stored in the mobile terminal, so as to detect thenumber of the error bits contained in the reference pattern dataportion; and an error correcting means for correcting a user dataportion in the received data and for detecting the number of thecorrected bits, wherein the bit error rate measuring means measures thebit error rate of the receive signal, on the basis of the number of theerror bits in the reference pattern data portion, which is measured bythe reference pattern comparing means, and the number of the correctedbits in the user data portion, which is detected by the error correctingmeans.